Oscilloscope Bandwidth Digital Applications
Experience tells us that the bandwidth of the oscilloscope should be at least 5 times greater than the fastest digital clock rate of the system under test. If the oscilloscope we choose meets this criterion, then the oscilloscope can capture the 5th harmonic of the signal under test with minimal signal attenuation. The 5th harmonic of a signal is very important in determining the overall shape of a digital signal. But if accurate measurements of high-speed edges are required, this simple formula does not take into account the actual high-frequency content contained in fast rising and falling edges.
Formula: fBW≥5xfclk
A more accurate way to determine an oscilloscope's bandwidth is based on the highest frequency present in the digital signal, rather than the maximum clock rate. The highest frequency of a digital signal depends on the fastest edge speed in the design. Therefore, we first determine the rise and fall times of the fastest signals in the design. This information can usually be obtained from the published specifications of the devices used in the design.
Use a simple formula to calculate the maximum "real" frequency content of a signal. Dr. Howard W. Johnson wrote a book "High-Speed Digital Design" on this topic. In the book, he calls this frequency component the "knee" frequency (fknee). All fast edges contain an infinite number of frequency components in their spectrum, but there is a point of inflection (or "knee") above which frequency components are insignificant in determining the shape of the signal. Step 2: Calculate fknee
fknee=0.5/RT(10%-90%)fknee=0.4/RT(20%-80%)
For a signal whose rise time characteristics are defined according to the 10% to 90% threshold, the knee frequency fknee is equal to 0.5 divided by the rise time of the signal. For a signal whose rise time characteristics are defined by a 20% to 80% threshold, as is often the case in today's device specifications, fknee is equal to 0.4 divided by the signal's rise time. But be careful not to confuse the signal rise time here with the oscilloscope's rise time specification. What we are talking about here is the actual signal edge speed. The third step is to determine the oscilloscope bandwidth required to measure this signal based on how accurately you need to measure the rise and fall times. Table 1 shows the relationship between the required oscilloscope bandwidth and fknee under various accuracy requirements for an oscilloscope with Gaussian frequency response or maximum flat frequency response. But keep in mind that most oscilloscopes with bandwidth specifications of 1GHz and below are usually Gaussian frequency response types, while those with bandwidths over 1GHz are usually maximum flat frequency response types. Table 1: Coefficients for calculating the required bandwidth of an oscilloscope based on the required accuracy and type of oscilloscope frequency response Step 3: Calculate the oscilloscope bandwidth
Let's explain it through a simple example:
For an oscilloscope to have a correct Gaussian frequency response when measuring a 500ps rise time (10-90%), determine the minimum bandwidth required; if the signal has a rise/fall time of approximately 500ps (defined by the 10% to 90% criterion ), then the maximum actual frequency component of the signal fknee=(0.5/500ps)=1GHz
If a 20% timing error is allowed when making rise and fall time parameter measurements, an oscilloscope with a bandwidth of 1 GHz will suffice for this digital measurement application. But if the timing accuracy is required to be within 3%, it is better to use an oscilloscope with a bandwidth of 2GHz.
20% timing accuracy: oscilloscope bandwidth=1.0x1GHz=1.0GHz
3% timing accuracy: oscilloscope bandwidth=1.9x1GHz=1.9GHz
